Remove usage of dbt.contracts.relation in dbt/adapters

[MichelleArk]

resolves #9208

Problem

usage of dbt.contracts.relation in dbt/adapters

Solution

Moved relation.py to adapters.contracts.relation - it was only being used in dbt/adapters so this was a straightforward lift.

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
• This PR includes type annotations for new and modified functions

[github-actions]

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[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (00f4920) 86.58% compared to head (2cee865) 86.59%.

Files	Patch %	Lines
core/dbt/common/exceptions/contracts.py	55.55%	4 Missing ⚠️

Additional details and impacted files

@@                           Coverage Diff                           @@
##           feature/decouple-adapters-from-core    #9207      +/-   ##
=======================================================================
+ Coverage                                86.58%   86.59%   +0.01%     
=======================================================================
  Files                                      214      215       +1     
  Lines                                    26885    26871      -14     
=======================================================================
- Hits                                     23278    23269       -9     
+ Misses                                    3607     3602       -5     

Flag	Coverage Δ
integration	83.48% <78.94%> (+<0.01%)	⬆️
unit	65.20% <78.94%> (-0.09%)	⬇️

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[MichelleArk]

After this refactor, the remaining core imports in dbt/adapters are:

core/dbt/adapters/protocol.py:
  - dbt.contracts.graph.nodes::ResultNode
  - dbt.contracts.graph.model_config::BaseConfig
  - dbt.contracts.graph.manifest::Manifest

core/dbt/adapters/factory.py:
  - dbt.mp_context::get_mp_context

core/dbt/adapters/base/relation.py:
  - dbt.contracts.graph.nodes::SourceDefinition
  - dbt.contracts.graph.nodes::ManifestNode
  - dbt.contracts.graph.nodes::ResultNode
  - dbt.contracts.graph.nodes::ParsedNode
  - dbt.node_types::NodeType

core/dbt/adapters/base/connections.py:
  - dbt.contracts.graph.manifest::Manifest

core/dbt/adapters/base/query_headers.py:
  - dbt.context.manifest::generate_query_header_context
  - dbt.contracts.graph.nodes::ResultNode
  - dbt.contracts.graph.manifest::Manifest

core/dbt/adapters/base/impl.py:
  - dbt.contracts.graph.manifest::Manifest
  - dbt.contracts.graph.manifest::MacroManifest
  - dbt.contracts.graph.nodes::ResultNode

migration progress: 93.44% of dbt imports are valid (from adapters or common)
remaining core imports: 16