Introduce Template Serializer for exposing metadata

What does this MR do?

• Display
• Search
• Use

project = Project.first
current_user = User.first
templates = TemplateFinder.new(:gitlab_ci_ymls, project, { metadata: { stages: ['devops::configure'] } }).execute
templates = TemplateFinder.new(:gitlab_ci_ymls, project, { metadata: { categories: ['Category:Container Scanning'] } }).execute

http://local.gitlab.test:8181/shinya/test-ci-yaml-templates/-/ci/editor/templates.json?stages[]=configure
http://local.gitlab.test:8181/shinya/test-ci-yaml-templates/-/ci/editor/templates.json?name[]=Auto
http://local.gitlab.test:8181/shinya/test-ci-yaml-templates/-/ci/editor/templates.json?maintainers[]=authoring&name[]=Bash

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Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• Label as security and @ mention @gitlab-com/gl-security/appsec
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