Taxonomy of optional capabilities for taps and targets

[aaronsteers]

(Partially enumerated in: https://gitlab.com/meltano/meltano/-/issues/2829 and in related SDK MR: https://gitlab.com/meltano/sdk/-/merge_requests/141/diffs#1a46f5bc80ac2afe420e39f5eb5c82cb4d690483_0_112)

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For taps only:

• discover - the ability to print a catalog.json file with all known streams and their schemas (as described in existing Singer Spec)
• catalog (newer) and properties (previous iteration) - the ability to pass in a catalog.json file with selection logic (as described in existing Singer Spec)
• log-based the ability for database-type sources to read from logs (as described in existing Singer Spec)

For targets only:

• soft-delete - The ability to flag records as deleted, based on a deleted_at property or some other signal.
• hard-delete - The ability to permanently delete records, based on a deleted_at property or some other signal.
• datatype-failsafe - The promise that records will be written to the target when the data type is not in an expected/handled set of cases and/or when the target cannot parse the JSON Schema type (as proposed in Best Practice Target Capability: Data Type Failsafe #20).

For both taps and targets:

• about - prints out capabilities and settings (as proposed in Discoverable capabilities and settings (--about support in CLI) #8)
• activate-version support (currently unofficial but widely used, as described in Standardization of ACTIVATE_VERSION message #9)
• batch support (future capability, as proposed in Spec Proposal: BATCH message type (aka FASTSYNC) #2)

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Related to: #4