Check RSE in _find method of RucioRemoteFrontend

[dachengx]

What does the code in this PR do / what does it improve?

Only being able to search the rules by admix.rucio.list_rules can not ensure the availability of did. We need to make sure the procedures to determine whether the data can be found are the same in RucioRemoteFrontend._find and RucioRemoteBackend._get_rse.

Can you briefly describe how it works?

Directly use RucioRemoteBackend._get_rse in RucioRemoteFrontend._find.

Can you give a minimal working example (or illustrate with a figure)?

Please include the following if applicable:

• Update the docstring(s)
• Update the documentation
• Tests to check the (new) code is working as desired.
• Does it solve one of the open issues on github?

Notes on testing

• Until the automated tests pass, please mark the PR as a draft.
• On the XENONnT fork we test with database access, on private forks there is no database access for security considerations.

All italic comments can be removed from this template.

[coveralls]

coverage: 90.219% (+0.02%) from 90.199%
when pulling 4d8ba1f on rucio_remote_frontend_find
into d78177f on master.

[dachengx]

This PR is following #1435