This change adds a User Agent for all http calls as part of source. This
will append the user agent being passed as part of prebuilt tools
context to default headers.
This commit refactors the source configuration and loading mechanism to
use a dynamic registration pattern. Each source package now registers
itself with a central registry via its init() function.
The server configuration code uses this registry to decode and
initialize sources, decoupling it from specific source implementations
and simplifying the addition of new sources.
Key changes:
- Introduced `sources.Register()` and `newConfig()` constructor in each
source package.
- Moved source package imports to `cmd/root.go` as blank imports to
trigger `init()` functions for self-registration.
- Removed direct imports of specific source packages from
`internal/server/config.go`.
- Renamed `SourceKind` constants to `Kind` within each source package.
- Updated tests to use the new `Kind` constants and reflect registration
changes.
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Co-authored-by: Yuan Teoh <yuanteoh@google.com>
