Provide more convenient mechanisms for setting common options

[cayhorstmann]

Multiple users report grief with configuring Eglot/JDTLS, partially because of bugs such as https://github.com/eclipse-jdtls/eclipse.jdt.ls/issues/3108, and partially because of the complexity of the configurations.

One pain point is setting the formatter. It would be nice to have configuration options similar to lsp-java, such as

eglot-java-format-settings-url
eglot-java-format-settings-profile

Another is setting compiler flags such as preview mode or release version. In JDTLS, these need to be put in a config file. There are so many of them that it is probably impractical to make them all individually settable, but one could have

eglot-java-settings-url

thereby raising awareness to this mechanism.

Then eglot-java could do the right thing to set those options. Right now, in light of https://github.com/eclipse-jdtls/eclipse.jdt.ls/issues/3108, one should use a default workspace configuration, not initialization options, which will be ignored.