feat: add support for multiplexed session for read only transactions #5509

Replace `·MultiplexedSessionOptions·|·MultiplexedSessionConstructor,·` with `⏎······|·MultiplexedSessionOptions⏎······|·MultiplexedSessionConstructor`

lint: src/database.ts#L370

Trailing spaces not allowed

lint: src/database.ts#L489

Replace `this,·name,·poolOptions,·multiplexedSessionOptions` with `⏎······this,⏎······name,⏎······poolOptions,⏎······multiplexedSessionOptions⏎····`

lint: src/database.ts#L1149

Replace `(process.env.GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS!==` with `·(process.env.GOOGLE_CLOUD_SPANNER_MULTIPLEXED_SESSIONS·!==·`

lint: src/database.ts#L3070

Replace `"session:·"` with `'session:·'`

lint: src/get-session.ts#L1

Replace `·Database,·Session,·Transaction·}·from·"."` with `Database,·Session,·Transaction}·from·'.'`

lint: src/get-session.ts#L2

Replace `·MultiplexedSession,·MultiplexedSessionInterface,·MultiplexedSessionOptions·}·from·"./multiplexed-session"` with `⏎··MultiplexedSession,⏎··MultiplexedSessionInterface,⏎··MultiplexedSessionOptions,⏎}·from·'./multiplexed-session'`

lint: src/get-session.ts#L3

Replace `·SessionPool,·SessionPoolInterface,·SessionPoolOptions·}·from·"./session-pool"` with `⏎··SessionPool,⏎··SessionPoolInterface,⏎··SessionPoolOptions,⏎}·from·'./session-pool'`

lint: src/get-session.ts#L4

Replace `·MultiplexedSessionConstructor,·SessionPoolConstructor·}·from·"./database"` with `⏎··MultiplexedSessionConstructor,⏎··SessionPoolConstructor,⏎}·from·'./database'`

test (20)

Process completed with exit code 22.

test (16)

The job was canceled because "_20" failed.

test (16)

The operation was canceled.

test (18)

The job was canceled because "_20" failed.

test (18)

The operation was canceled.

test (14)

The job was canceled because "_20" failed.