Added a mapping for DPOP TokenType in DefaultMapOAuth2AccessTokenResponseConverter

[hammadirshad]

This PR adds the missing mapping for the newly introduced TokenType.DPOP in DefaultMapOAuth2AccessTokenResponseConverter. Without this mapping, deserializing an OAuth 2.0 Access Token Response containing a DPoP token type results in an error:
An error occurred reading the OAuth 2.0 Access Token Response: tokenType cannot be null

Stacktrace

org.springframework.web.client.RestClientException: Error while extracting response for type [org.springframework.security.oauth2.core.endpoint.OAuth2AccessTokenResponse] and content type [application/json;charset=UTF-8]
	at org.springframework.web.client.DefaultRestClient.readWithMessageConverters(DefaultRestClient.java:261) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.DefaultRestClient$DefaultResponseSpec.readBody(DefaultRestClient.java:814) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.DefaultRestClient$DefaultResponseSpec.lambda$body$0(DefaultRestClient.java:745) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.exchangeInternal(DefaultRestClient.java:574) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.DefaultRestClient$DefaultRequestBodyUriSpec.exchange(DefaultRestClient.java:535) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.RestClient$RequestHeadersSpec.exchange(RestClient.java:677) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.DefaultRestClient$DefaultResponseSpec.executeAndExtract(DefaultRestClient.java:809) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.DefaultRestClient$DefaultResponseSpec.body(DefaultRestClient.java:745) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.security.oauth2.client.endpoint.AbstractRestClientOAuth2AccessTokenResponseClient.getTokenResponse(AbstractRestClientOAuth2AccessTokenResponseClient.java:94) ~[spring-security-oauth2-client-6.5.0-M3.jar:6.5.0-M3]
	... 25 common frames omitted
Caused by: org.springframework.http.converter.HttpMessageNotReadableException: An error occurred reading the OAuth 2.0 Access Token Response: tokenType cannot be null
	at org.springframework.security.oauth2.core.http.converter.OAuth2AccessTokenResponseHttpMessageConverter.readInternal(OAuth2AccessTokenResponseHttpMessageConverter.java:81) ~[spring-security-oauth2-core-6.5.0-M3.jar:6.5.0-M3]
	at org.springframework.security.oauth2.core.http.converter.OAuth2AccessTokenResponseHttpMessageConverter.readInternal(OAuth2AccessTokenResponseHttpMessageConverter.java:47) ~[spring-security-oauth2-core-6.5.0-M3.jar:6.5.0-M3]
	at org.springframework.http.converter.AbstractHttpMessageConverter.read(AbstractHttpMessageConverter.java:198) ~[spring-web-6.2.5.jar:6.2.5]
	at org.springframework.web.client.DefaultRestClient.readWithMessageConverters(DefaultRestClient.java:244) ~[spring-web-6.2.5.jar:6.2.5]
	... 33 common frames omitted
Caused by: java.lang.IllegalArgumentException: tokenType cannot be null
	at org.springframework.util.Assert.notNull(Assert.java:181) ~[spring-core-6.2.5.jar:6.2.5]
	at org.springframework.security.oauth2.core.OAuth2AccessToken.<init>(OAuth2AccessToken.java:76) ~[spring-security-oauth2-core-6.5.0-M3.jar:6.5.0-M3]
	at org.springframework.security.oauth2.core.endpoint.OAuth2AccessTokenResponse$Builder.build(OAuth2AccessTokenResponse.java:191) ~[spring-security-oauth2-core-6.5.0-M3.jar:na]
	at org.springframework.security.oauth2.core.endpoint.DefaultMapOAuth2AccessTokenResponseConverter.convert(DefaultMapOAuth2AccessTokenResponseConverter.java:64) ~[classes/:na]
	at org.springframework.security.oauth2.core.endpoint.DefaultMapOAuth2AccessTokenResponseConverter.convert(DefaultMapOAuth2AccessTokenResponseConverter.java:37) ~[classes/:na]
	at org.springframework.security.oauth2.core.http.converter.OAuth2AccessTokenResponseHttpMessageConverter.readInternal(OAuth2AccessTokenResponseHttpMessageConverter.java:77) ~[spring-security-oauth2-core-6.5.0-M3.jar:6.5.0-M3]
	... 36 common frames omitted