Add logic to return channels to FakeBackendV2

[nkanazawa1989]

Summary

This PR adds a logic to FakeBackendV2 that parses the reported channel mapping and returns the pulse channels with method call, such as .drive_channel(qubit). Because the corresponding V1 backend returns the channels, this is kind of a bug due to missing logic.

Details and comments

Fix #7832

[qiskit-bot]

Thank you for opening a new pull request.

Before your PR can be merged it will first need to pass continuous integration tests and be reviewed. Sometimes the review process can be slow, so please be patient.

While you're waiting, please feel free to review other open PRs. While only a subset of people are authorized to approve pull requests for merging, everyone is encouraged to review open pull requests. Doing reviews helps reduce the burden on the core team and helps make the project's code better for everyone.

One or more of the the following people are requested to review this:

• @Qiskit/terra-core

[mtreinish]

This has partial overlap with #8318 although this covers more of the optional attributes in the BackendV2 interface. @gadial are you ok with closing #8318 in favor of this? Or do you want to take @nkanazawa1989's additions into #8318 too

[nkanazawa1989]

The question is if we can instantiate PulseBackendConfiguration with V2. Seems like this approach is overkill to just parse the channels mapping, i.e. PulseBackendConfiguration._parse_channels.

[gadial]

This has partial overlap with #8318 although this covers more of the optional attributes in the BackendV2 interface. @gadial are you ok with closing #8318 in favor of this? Or do you want to take @nkanazawa1989's additions into #8318 too

I think Naoki's PR is better since it is more focused; I intend to close #8318 .

[coveralls]

Pull Request Test Coverage Report for Build 2907173820

• 38 of 43 (88.37%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.004%) to 84.042%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/providers/fake_provider/fake_backend.py	38	43	88.37%

Files with Coverage Reduction	New Missed Lines	%
qiskit/extensions/quantum_initializer/squ.py	2	79.78%

Totals
Change from base Build 2907171101:	0.004%
Covered Lines:	56260
Relevant Lines:	66943

---

💛 - Coveralls

[mtreinish]

LGTM, one inline comment/question but not a blocker.

[mtreinish]

Not that regex time here will be a big deal but do you think it'll be worthwhile to move this to a class level variable so we don't have to have the compile time on each instantiation of a FakeBackendV2?