feat: Allow for attaching metadata and pass it to the API and transports

[kamilogorek]

Still need some tinkering and tests.

• allow for passing options._metadata which is type of SdkMetadata = { sdk?: SdkInfo }
• this metadata is then attached to options.transportOptions internally and passed to the transport
• transport is using this metadata when instantiating new API which now has the ability to read this data
  - SDK_VERSION is fixed and always read from @sentry/core
• SDK_VERSION is moved to @sentry/core
• SDK_NAME is kept where it was for backwards compatibility and @sentry/browser and @sentry/node still use it as before, so it can be used as fallback for "unknown" SDKs or in a fail-safe scenario
  - event processors are still in place, as we use them to enhance packagesattribute, as well as we have to modify the event itself, which is not using envelopes yet

[github-actions]

size-limit report

Path	Size
@sentry/browser - CDN Bundle (gzipped)	20.16 KB (+0.81% 🔺)
@sentry/browser - Webpack	20.97 KB (+0.66% 🔺)
@sentry/react - Webpack	20.99 KB (+0.78% 🔺)
@sentry/browser + @sentry/tracing - CDN Bundle (gzipped)	27.31 KB (+0.59% 🔺)

[rhcarvalho]

For posteriority, this is related to #3170, as it would simplify the implementation of that one.

[rhcarvalho]

Oh I like that eventually we can have the version defined in a single place ❤️

[lobsterkatie]

For posteriority, this is related to #3170, as it would simplify the implementation of that one.

Indeed. @kamilogorek, even though this replaces that one, you might consider grabbing its tests (which you'd only have to modify a little) since they're already written. (I missed gatsby entirely in that PR, but fortunately the test there already exists and only needs to be updated.)

[HazAT]

We only want name and version here not everything.

[HazAT]

Same here: We only want name and version here, not everything.