loaders: allow same importer through multiple parsers

[kecnry]

Description

This pull request allows the user to choose between loader paths through multiple parsers by including the parser in the format. For example, if both are valid, the user can choose between fits > 2D Spectrum (with options for extension, transposing etc) and specutils.Spectrum > 2D Spectrum.

This however still allows setting (or passing) format as just the importer. So viz.load(filename, format='2D Spectrum') is allowed. Currently this does not require a single unique match, but we could change that behavior to require passing the parser if multiple format options are matched with the passed format.

This PR also tracks the reason that loaders were unsuccessful for easier debugging when inputs are unsuccessful.

Change log entry

• [x] Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts, list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• [ ] Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• [ ] Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• [ ] Do the proposed changes follow the STScI Style Guides?
• [ ] Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• [ ] Did the CI pass? If not, are the failures related?
• [x] Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone. Bugfix milestone also needs an accompanying backport label.
• [ ] After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?