R2DBC: Support Google Cloud Spanner

[hfhbd]

Description

I tried to use Google Cloud Spanner with SqlDelight: https://github.com/hfhbd/sqldelight-spanner-r2dbc
There are 4 problems, two of them are needed to fix somehow in sqldelight:

1. Known problem with R2DBC driver:
   Spanner R2DBC requires @ as a parameter identifier, no $ nor ? at runtime. So again, we need to generate/replace these identifiers somehow.

2. Spanner does not support bind(index, value), this throw a UnsupportedOperationException. Instead, you have to use the name of the column: bind(name, value).

Other "problems" unrelated to sqldelight, there is no native Spanner sqldelight dialect and to test the linked demo, a new release of Spanner driver is required (1.2.1)

[hfhbd]

One option: Split compiler-core into "plugins" so overwriting parts of the compiler is possible. Spanner would implement the query generator and change the relevant parts (named binding and @)