Include check for rx gates for fractional gates support

[jan-an]

• Fixes QiskitRuntimeService should also check for rx in the backend basis when fractional gates=True #2520
• QiskitRuntimeService.backend also checks for Rx gates in the backend basis when setting use_fractional_gates=True for input validation (commit)
• Test updated (commit)
• Release notes added (commit)

[yaelbh]

Thanks @jan-an.
Note that another bug will arise if you run backend with use_fractional_gates=True, for a backend that has only rx and no rzz, then afterwards call backend again with the same backend name but use_fractional=False - a cached configuration will be taken instead of a refreshed one (this is the line). Similarly, the line before will cause the configuration to be refreshed again and again for repeated calls to backend with use_fractional=True, this is not a functional bug but a performance one. I'll open an issue about these bugs.

[jan-an]

Thanks @jan-an. Note that another bug will arise if you run backend with use_fractional_gates=True, for a backend that has only rx and no rzz, then afterwards call backend again with the same backend name but use_fractional=False - a cached configuration will be taken instead of a refreshed one (this is the line). Similarly, the line before will cause the configuration to be refreshed again and again for repeated calls to backend with use_fractional=True, this is not a functional bug but a performance one. I'll open an issue about these bugs.

Hi @yaelbh, thanks a lot for pointing out this problem! Would you like me to address anything related to that as part of this PR?

[yaelbh]

This PR can be merged, anything else can be done in other PRs.