Support converting other config files to Flow files

[rfratto]

As Flow gains adoption, supporting config conversions from various tools which have rough feature parity with Flow will help users migrate to Flow.

Conversion should be best-effort, if something configured in the source config does not have an equivalent in Flow, the conversion should generate either a warning or an error.

Conversion should be defined for anything that exists in Flow which has a reasonable equivalent in the source config file.

After conversion exists, we should update the PR template to remind authors to update the conversion code, if relevant to the new component.

Tasks

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1. Create Convert API and the internal pattern for the API #3736
   flow frozen-due-to-age +2
   
2. Implement a way to pass capsules from 1 component to another in config conversion to Flow #3902
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3. Allow outputing of secrets as string when doing config conversion to Flow #3901
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4. Implement a way to warn users if unsupported config is provided #3917
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5. Introduce new interface for managing river defaults and validation #3988
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6. Don't include defaults in the output converted config if the value matches the Flow default #3900
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7. Wire up the Convert API to the grafana-agent command line for translating supported configs to river #3737
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8. Wire up the Convert API to the run command for translating supported configs to river on the fly #3738
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9. Add documentation for using the config conversion tools for end users #4073
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10. Support converting Prometheus configs to supported Flow configs #4018
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11. Add support for converting OpenTelemetry Collector components #6419
    40 of 40
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12. Support converting Promtail configs to support Flow configs #4292
    flow frozen-due-to-age +2
    
13. Support converting supported Grafana Agent Static mode configs to supported Flow configs #4291
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14. Add config conversion info to usage stats when runtime conversion is used #4444
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15. Build converter for static mode tracing subsystem #6591
    8 of 8
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