Swift: introduce application credential auth support

[kayrus]

What is the purpose of this change? What does it change?

Add Openstack Auth V3 application credential support.

Was the change discussed in an issue or in the forum before?

#2155

Manual testing could be performed following the steps below:

App cred could be created using the command below (openstack CLI version should be not lower than 3.15):

$ openstack application credential create --secret mysupersecretkey --role swiftoperator --restricted restic

Then, export env variables:

export OS_AUTH_URL="https://keystone/v3"
export OS_APPLICATION_CREDENTIAL_ID=%ID%
export OS_APPLICATION_CREDENTIAL_SECRET=mysupersecretkey

or

export OS_AUTH_URL="https://keystone/v3"
export OS_USERNAME=openstack_username
export OS_USER_DOMAIN_NAME=openstack_domain
export OS_APPLICATION_CREDENTIAL_NAME=restic
export OS_APPLICATION_CREDENTIAL_SECRET=mysupersecretkey

Checklist

• I have read the Contribution Guidelines
• I haven't added tests for all changes in this PR, since I don't have a possibility to create an application credential in test env
• I have added documentation for the changes (in the manual)
• There's a new file in changelog/unreleased/ that describes the changes for our users (template here)
• I have run gofmt on the code in all commits
• All commit messages are formatted in the same style as the other commits in the repo
• I'm done, this Pull Request is ready for review

[codecov-io]

Codecov Report

Merging #2156 into master will decrease coverage by 4.02%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #2156      +/-   ##
==========================================
- Coverage   50.76%   46.74%   -4.03%     
==========================================
  Files         176      176              
  Lines       14251    14257       +6     
==========================================
- Hits         7235     6664     -571     
- Misses       5962     6587     +625     
+ Partials     1054     1006      -48

Impacted Files	Coverage Δ
internal/backend/swift/config.go	34.69% <0%> (-56.62%)	⬇️
internal/backend/swift/swift.go	0% <0%> (-78.45%)	⬇️
internal/backend/b2/b2.go	0% <0%> (-80.69%)	⬇️
internal/backend/gs/gs.go	0% <0%> (-74%)	⬇️
internal/backend/azure/azure.go	0% <0%> (-69.46%)	⬇️
internal/archiver/blob_saver.go	95.06% <0%> (-4.94%)	⬇️
internal/archiver/file_saver.go	88.59% <0%> (+2.63%)	⬆️
internal/checker/checker.go	68.4% <0%> (+3.86%)	⬆️

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[kayrus]

@fd0, may I ask you to take a look on this PR? Thanks in advance.

[kayrus]

rebased on master

[kayrus]

@fd0 kindly ping

[fd0]

LGTM, although I don't have any means to test this change.