[TT-277] - Enhance tyk-sync to support filtering of APIs and policies

Description

If the user sets a --categories flag, Sync will look for APIs that match those categories If the user sets a --tags flag, Sync will look for APIs and Policies that match those tags Also, the readability of the Dump function has been improved, by separating code into blocks of functions.

Motivation and Context

Now users can look for APIs and Policies based on tags and categories, not only on IDs.

How This Has Been Tested

Unit tests were created.

Screenshots (if appropriate)

Types of changes

[ ] Bug fix (non-breaking change which fixes an issue)
[x] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Refactoring or add test (improvements in base code or adds test coverage to functionality)

Checklist

[x] Make sure you are requesting to pull a topic/feature/bugfix branch (right side). If pulling from your own fork, don't request your master!
[x] Make sure you are making a pull request against the master branch (left side). Also, you should start your branch off our latest master.
[ ] My change requires a change to the documentation.
- [ ] If you've changed APIs, describe what needs to be updated in the documentation.
- [ ] If new config option added, ensure that it can be set via ENV variable
[ ] I have updated the documentation accordingly.
[ ] Modules and vendor dependencies have been updated; run go mod tidy && go mod vendor
[ ] When updating library version must provide reason/explanation for this update.
[x] I have added tests to cover my changes.
[ ] All new and existing tests passed.
[ ] Check your code additions will not fail linting checks:
- [ ] go fmt -s
- [ ] go vet

TykTechnologies / tyk-sync

[TT-277] - Enhance tyk-sync to support filtering of APIs and policies #100