add base MicrobatchConcurrency capability

[MichelleArk]

resolves #359
docs dbt-labs/docs.getdbt.com/#

Problem

We'd like a mechanism for adapter maintainers to signal opt-in support for running the microbatch strategy concurrently.

Solution

Introduce a new Capability - MicrobatchConcurrency

Checklist

• I have read the contributing guide and understand what's expected of me
• I have run this code in development, and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX

Tested via: dbt-labs/dbt-core#10958

[github-actions]

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