Updating to support linq2db 2.5

[MaceWindu]

• [x] updated linq2db to 2.5.0
• [x] load SAP HANA provider from GAC instead of using local copy. This will fix #5. We don't need local copy anymore. Load from GAC was fixed in linq2db 2.1.0 and local copy not needed anymore
• [x] add aggregate functions support
• [x] add postgresql functions support
• [x] linq2db dependency should be bumped to 2.5.1 for this fix: https://github.com/linq2db/linq2db/pull/1382
• [x] port recent t4 fixes like C# keywords
• [x] add .editorconfig
• [x] fix ODP.NET provider discovery (needs this PR: https://github.com/linq2db/linq2db/pull/1384)
• [x] fixed 'Include Stored Procedures and Functions' checkbox handling - now procedures will be loaded only when it is checked
• [x] ~add workaround for SAP HANA procedures schema provider to not cause asserts from provider~ - decided to not fix it for now. If people find it annoying - we can fix it

[saberabdelhady]

System.TypeInitializationException HResult=0x80131534 Message=The type initializer for 'Sap.Data.Hana.HanaConnection' threw an exception. Source= StackTrace:

Inner Exception 1: Exception: The specified module could not be found

[sdanyliv]

@saberabdelhady, open an issue with detailed information.

[MaceWindu]

Note that you should use latest version of linqpad (v3) and install latest sap hana drivers (feb.2020) as SAP HANA drivers are very buggy and recent version contains better support for them.