[2.9 updates] New custom XCom backend content

[TJaniF]

Blocked by: Common IO provider 1.3.1 release Note to self: double check xcombackend class name, config values and provider versions before merging

Closes: #1874

• changes the custom XCom backend tutorial to use Object Storage
• moves the content about different XCom backend options to a new guide. I removed quite a bit from the original tutorial, Astronomer Providers are deprecated and there are other preferred methods to create custom serialization rules for different objects. (This could be its own guide eventually, since the OSS docs are quite short on this).

NOTE: GCP Object Storage XCom backend will depend on the next Google provider release, there is currently a bug there that will be fixed then.