feat: support customer managed instance configurations (#1611)

* feat: support customer managed instance configurations

* feat: add optional variables to requests

* feat: add support for listInstanceConfigOperations,

* docs: add api documentation

* docs: edit api doc.

* feat: change variable name.

* feat: fix typo in error.

* test: add cmmr phase 2 unit tests for index.ts

* Update src/index.ts

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

* Update src/index.ts

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

* Update src/instance-config.ts

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

* Update src/instance-config.ts

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

* Update src/index.ts

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

* feat: incorporate review comments and doc

* test: add unit tests and system tests.

-   unit tests for instance-config.ts
-   system tests for CMMR Phase 2

* test: add list instanceconfig operations integration test

* feat: set default value for gaxOpts

* feat: add some more methods for getInstanceConfigOperations

* test: system tests for cmmr phase 2

* feat: incorporate review comments.

* Update test/instance-config.ts

Co-authored-by: Knut Olav Løite <koloite@gmail.com>

* fix: fix property does not exist errors

* fix: err object type

* fix: headers

* fix: lint

* fix: lint

* fix: lint

* fix: replace === by == for null/undefined handling

* fix: linting

* feat: change base instance config

* fix: docs

* Revert "fix: docs"

This reverts commit ee87511.

* fix: docs

* fix: system test

* fix: system test

* fix: system test

* fix: corrected filter

* 🦉 Updates from OwlBot post-processor

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Co-authored-by: Knut Olav Løite <koloite@gmail.com>
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