feature: support DbString in DapperAOT

[DeagleGross]

DbString is a known type in Dapper and currently there is no dedicated support in DapperAOT for it.

Current PR does 2 things regarding DbString:

1. in analyzer mode reports DAP48 and suggests to move from usage of DbString to DbValue attribute.
2. properly configures DbParameter if one of parameter members is DbString by setting .Value, .Size and .DbType

Implementation notes:

1. I have moved files in Dapper.AOT.Analyzers which are copied to the output in separate folder InGeneration
2. I have created a single code writer for such types (PreGeneratedCodeWriter), and included the generation context options in GenerateState to have a control over which pre-generated types to include in the output. I decided to do in order to avoid writing DbStringHelpers in the output even in those cases, where DbString is not even used (basically to have more clear output)

Questions:

1. I am pretty sure I messed up on how to get the size of DbString in ANSI encoding. How do we do it properly?
2. What exactly do we need to do with the DbString in UpdateParameters? Do we need to call the same setup there, or just re-assigning value is fine?

Closes #89

[DeagleGross]

add Integration tests for dbstring (in a separate project Dapper.AOT.Test.Integration)

• dapper.vanilla without any interception (no dapper aot)
• dapper code with dapper aot with dbString
• dbvalue (interception)

[mgravell]

This is looking great; there's a ton of work in here, thanks; a couple of minor optional nits, but: massive thanks

[DeagleGross]

Hey @mgravell,
I know you asked me to implement integration tests - I tried but I still dont have a solution yet. Filed an issue to do it in a separate PR: #122

regarding testing DbString functionality: I packed Dapper.AOT, installed in local NuGet feed and used in a console app:

1. I can confirm that we emit a proposition to migrate from DbString to [DbValue] (only in Dapper.AOT mode)
   [module: DapperAot]
   

2. confirmed that query against local database works and returns the model I have in database (note: we hit the breakpoint in the generated DbStringHelpers - look at screenshot):
   

Repro of DbString usage