[FLINK-6811][table] Add TIMESTAMPADD supported in SQL

[sunjincheng121]

In this PR I have add TIMESTAMPADD supported in SQL. (the semantics keep consistent with calcite)

• Syntax
  timestampAdd (datepart SqlTypeFamily.ANY, number:SqlTypeFamily.INTEGER, date:SqlTypeFamily.TIMESTAMP )
  -datepart
  Is the part of date to which an integer number is added.
  -number
  Is an expression that can be resolved to an int that is added to a datepart of date
  -date
  Is an expression that can be resolved to a time.

• Example
  SELECT timestampAdd(month, 1, '2017-05-31') from tab; --> 2017-06-30 00:00:00.000

• Note: Due to the difference of [[org.apache.calcite.rex.Rex Literal]] between calcite 1.12 and calcite master we should temp close support the construce of TIMESTAMPADD(SqlTypeFamily.ANY, SqlTypeFamily.INTEGER, SqlTypeFamily.DATE), until upgrade to calcite 1.13. See more https://issues.apache.org/jira/browse/FLINK-6851

• General

  • The pull request references the related JIRA issue ("[FLINK-6811][table] Add TIMESTAMPADD supported in SQL")
  • The pull request addresses only one issue
  • Each commit in the PR has a meaningful commit message (including the JIRA id)

• Documentation

  • Documentation has been added for new functionality
  • Old documentation affected by the pull request has been updated
  • JavaDoc for public methods has been added

• Tests & Build

  • Functionality added by the pull request is covered by tests
  • mvn clean verify has been executed successfully locally or a Travis build has passed

[sunjincheng121]

Rebase code and add doc after FLINK-6960 & FLINK-6925.

[sunjincheng121]

I have rebase the code and update the PR.

[twalthr]

Thanks @sunjincheng121. Looks good to merge. I will merge this...