feat: Support blob type sources and GCS as an example of such source.

[alan-kut]

This PR

• adds support for blob type sources
• adds support for GCS using blob source

Notes

In a similar fashion other cloud storages supported by go-cloud blob can be added.

How to test

• create a Google Cloud Storage bucket and a file with flags definition inside
• get application default credentials: https://cloud.google.com/docs/authentication/provide-credentials-adc
• start the flagd with source pointing to the file (gs://...) and provider set to "gcs".

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[beeme1mr]

Hey @alan-kut, thanks for the PR. I'll test this out E2E tomorrow.

[alan-kut]

Thanks. I fixed lint error and data race.

Few assumptions I made in this code:

• It will return error from Sync and Resync if the blob cannot be accessed (authorization error, doesn't exist or other problems)
• It will log warning when sync on cron cannot access the blob
• It won't fetch blob and send updated data if the updated time of the blob hasn't changed (except for ReSync)

[alan-kut]

I changed the log to debug, signed off commit, updated the docs (in a new commit).

Should I squash the commits somehow? Rebase them on main?

[beeme1mr]

I've reviewed all the licensing issues detected by FOSSA. The library is licensed BSD-3, but comments in the code referred to other licenses, which triggered a false positive.

[toddbaert]

I changed the log to debug, signed off commit, updated the docs (in a new commit).

Should I squash the commits somehow? Rebase them on main?

Commits are squashed automatically, everything will be clean in main.