Document the supported Flyway bootstrap for jdbc:libsql: URLs #263

Goal

Document, and cover with a test, how to point Flyway at a libSQL database:
pass a DataSource, or use URL configuration with the jdbc:sqlite: alias and
an explicit driver class.

Why

Flyway resolves its database plugin from the URL before it opens a connection,
and it never asks DriverManager which driver accepts the URL. As a result:

• jdbc:libsql: is not claimed by Flyway's SQLite plugin, which accepts only
  jdbc:sqlite:, jdbc:sqldroid: and the p6spy prefixes:

  No Flyway database plugin found to handle jdbc:libsql:********
  

• jdbc:sqlite: is claimed, but Flyway's plugin resolves the driver to
  org.sqlite.JDBC. Programmatic Flyway.configure().dataSource(url, ...)
  builds the DriverDataSource eagerly and passes a null driver class, so a
  later driver(...) is ignored:

  No JDBC driver found to handle jdbc:sqlite:********
  

This is not fixable inside the driver. It is a configuration and documentation
gap.

Workarounds that work today

• DataSource (Spring Boot, HikariCP, any pool) with either jdbc:libsql:
  or jdbc:sqlite:. Pinned by
  FlywayMigrationIT#migratesThroughADataSource.
• URL + explicit driver with the jdbc:sqlite: alias, for example
  Flyway.configure(new ClassicConfiguration()) with setUrl(...) and
  setDriver("io.github.magnusp.libsql.jdbc.LibsqlDriver"), or the equivalent
  Flyway Maven plugin / flyway.properties url and driver settings. Pinned
  by FlywayMigrationIT#migrateWithUrlConfigurationAndExplicitDriver.
• jdbc:libsql: cannot be used with URL-based configuration at all, even with
  an explicit driver, because the plugin lookup fails first. Pinned by
  FlywayKnownGapsIT#libsqlUrlIsNotRecognizedByFlyway.

Session-state note

Flyway's initSql / afterConnect cannot tune connection-scoped state such as
PRAGMA foreign_keys: each auto-commit statement runs on a fresh Hrana stream
(see #83). Configure such state in the DataSource/pool, or run the statements in
an explicit transaction. The Flyway clean() consequence is tracked in #258.

Where

• Add a Flyway section to docs/ (or extend README.md) with the working
  configurations above.
• Keep the two negative pins in FlywayKnownGapsIT.
• Cross-link Keep a connection-scoped Hrana stream for auto-commit: fix last_insert_rowid() and Flyway clean() #258 for clean() support.

Verification

• The documented snippets run against the pinned sqld container.
• ./mvnw -pl libsql-jdbc -am verify -Dit.test='Flyway*IT'
• ./mvnw spotless:check

Found while adding the Flyway integration tests in #259.