Support serverinfo of lsp 3.15.0

Support serverinfo of lsp 3.15.0

Add support for serverInfo from the upcoming specification.  This
changeset just stores the info sent by the server and slightly changes
a greeting message.  But it opens up the possibility to identify
servers even when eglot uses a TCP connection and therefore makes
possible to implement server specific features (in eglot-x).

Old message:
```
Connected! Server `EGLOT (test-ccls/c++-mode)' now managing `c++-mode' buffers in project `test-ccls'.
```

New message:
```
Connected! Server `ccls' now managing `c++-mode' buffers in project `test-ccls'.
```

* eglot.el (eglot--lsp-interface-alist):  Extend it with serverInfo.
(eglot-lsp-server): Add member variable server-info.
(eglot--connect): Store server-info and display server's name
when connected.