Support table alias in new engine

[dai-chen]

Issue #, if available:

1. SELECT <index>.<column> AS alias is not supported #242: alias.`field` parsing issue

Description of changes: This PR is to support table alias in FROM clause, FROM index idx.

1. Relation: An optional alias field is added. It's always null for PPL which doesn't support table alias
2. Analyzer: When analyzing relation, its name (or alias if present) will be pushed to a new INDEX namespace on type environment.
3. ExpressionAnalyzer and SelectExpressionAnalyzer: A new abstraction QualifierAnalyzer is added to resolve qualifier in index namespace. Un-qualify the qualified name by removing the qualifier if found. Or throw exception if qualifier doesn't exist or not an index.

Documentation:

1. Specification and more details about identifier qualifiers: https://github.com/dai-chen/sql/blob/support-table-alias-in-new-engine/docs/user/general/identifiers.rst#identifier-qualifiers
2. Limitation: https://github.com/dai-chen/sql/blob/support-table-alias-in-new-engine/docs/user/limitations/limitations.rst#limitations-on-identifiers

Testing: new UT, IT (comparison test) and IT for the #242 bug fix.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[penghuo]

LGTM