Support oracle dialect

[long2ice]

Description

Motivation and Context

How Has This Been Tested?

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[coveralls]

Pull Request Test Coverage Report for Build 2166289660

• 26 of 245 (10.61%) changed or added relevant lines in 10 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-1.6%) to 86.13%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
tortoise/backends/odbc/init.py	0	2	0.0%
tortoise/backends/oracle/init.py	0	2	0.0%
tortoise/backends/mssql/client.py	0	6	0.0%
tortoise/backends/oracle/executor.py	0	10	0.0%
tortoise/backends/oracle/client.py	0	51	0.0%
tortoise/backends/oracle/schema_generator.py	0	63	0.0%
tortoise/backends/odbc/client.py	0	85	0.0%

Files with Coverage Reduction	New Missed Lines	%
tortoise/backends/mssql/client.py	1	0%

Totals
Change from base Build 2161406731:	-1.6%
Covered Lines:	5321
Relevant Lines:	6155

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💛 - Coveralls