Add API to open TCP based TPM device

https://github.com/stefanberger/swtpm/ is a TPM software emulator. One of the modes it provides is a TCP communication channel. The advantage of using a TCP connection is that it does not require sudo permissions to create a new emulated device, and it is something important for unit testsing.

Some tools (e.g. IBM TSS) allow to use such socket connection by setting special envvar, see https://github.com/stefanberger/swtpm/wiki/Using-the-IBM-TSS-with-swtpm for example.

It might be handy if `go-tpm` in addition to API that deals with filesystem paths (`tpm2.OpenTPM(filepath)`) also had an ability to open a TCP-based communication channel.

Start by reading the existing tpm2.OpenTPM(filepath) API and the linked swtpm TCP communication documentation. Determine how a TCP-based TPM connection should fit the current API, then verify that the resulting API can connect to an emulated device without filesystem access.

Written by the indexing model from the issue text.