Use LSP for pom editor

[tsmaeder]

What does this PR do?

This PR removes the custom Maven pom editor and uses the LSP-based editor instead. In order to achieve this, it introduces a Maven language server that runs in the ws-agent process. Maven problems are communicated as LSP diagnostics instead of custom problem instances.
The reason for this PR is to open up the pom editor for extension by additional language servers, in particular, the bayesian language server on OSIO.

What issues does this PR fix or reference?

#5492

Changelog

Use language server protocol editor for pom.xml files

Release Notes

This should be transparent to the user.

Docs PR

No change

[codenvy-ci]

Can one of the admins verify this patch?

[codenvy-ci]

Can one of the admins verify this patch?

[riuvshin]

@tsmaeder this changes broke build https://ci.codenvycorp.com/job/che-ci-master/1552/console

20:39:11 /home/codenvy/workspace/che-ci-master/plugins/plugin-maven/che-plugin-maven-server/src/main/java/org/eclipse/che/plugin/maven/lsp/MavenTextDocumentService.java:33: error: MavenTextDocumentService is not abstract and does not override abstract method completion(TextDocumentPositionParams) in TextDocumentService
20:39:11 public class MavenTextDocumentService implements TextDocumentService {
20:39:11        ^
20:39:11 /home/codenvy/workspace/che-ci-master/plugins/plugin-maven/che-plugin-maven-server/src/main/java/org/eclipse/che/plugin/maven/lsp/MavenTextDocumentService.java:41: error: completion(TextDocumentPositionParams) in MavenTextDocumentService cannot implement completion(TextDocumentPositionParams) in TextDocumentService
20:39:11     public CompletableFuture<CompletionList> completion(TextDocumentPositionParams position) {
20:39:11                                              ^
20:39:11   return type CompletableFuture<CompletionList> is not compatible with CompletableFuture<Either<List<CompletionItem>,CompletionList>>
20:39:11 /home/codenvy/workspace/che-ci-master/plugins/plugin-maven/che-plugin-maven-server/src/main/java/org/eclipse/che/plugin/maven/lsp/MavenTextDocumentService.java:40: error: method does not override or implement a method from a supertype
20:39:11     @Override
20:39:11     ^