Add optional headers to DownloadableFile messages when downloading files

[RichardChukwu]

Fixes #299

This PR implements the recent specification change that allows for optional headers in the DownloadableFile message, which can be used during file downloads to support authentication and other header-based configurations.

The downloadFile function in the packagesSyncer struct has been modified to include optional headers from the DownloadableFile message. This enhancement allows Agents to set custom HTTP headers when downloading files from the server. For instance, headers such as Authorization can now be included to facilitate secure downloads.

Changes Made:

-Updated the downloadFile method to check for the presence of optional headers in the DownloadableFile message.
-Added logic to incorporate these headers into the HTTP request when downloading files.
-Ensured compatibility with the updated protocol specification that outlines the use of headers for authentication and other purposes.

[tigrannajaryan]

The build fails because we need to update to the new version of Protobufs that have the new Headers field.

This PR does it #307

[tigrannajaryan]

Please rebase, it should fix the build.

[RichardChukwu]

Rebase done @tigrannajaryan

[tigrannajaryan]

Please fix the build: https://github.com/open-telemetry/opamp-go/actions/runs/11499942257/job/32122224014?pr=306

[srikanthccv]

@RichardChukwu ICYMI, the test failure logs.

Error: client/clientimpl_test.go:1511:17: undefined: createDownloadSrv
Error: client/clientimpl_test.go:1660:17: undefined: createDownloadSrv

you can verify the changes locally by running make test

[RichardChukwu]

srikanthccv

Thanks, working on it already

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@36a1afd). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
client/internal/packagessyncer.go	0.00%	3 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #306   +/-   ##
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  Coverage        ?   77.11%           
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  Files           ?       25           
  Lines           ?     2307           
  Branches        ?        0           
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  Hits            ?     1779           
  Misses          ?      423           
  Partials        ?      105           

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