configurable jvm_args & workspace for jdtls server

Is your feature request related to a problem? Please describe.

lspconfig allows jvm_args and workspace to be overridden by setting config.init_options.

nvim-lsp-installer seems to be using config.vmargs & vim.env.WORKSPACE.

Since nvim-lsp-installer allows to be setup through the regular lspconfig.setup() it would be nice to respect lspconfig config.

Describe the solution you'd like

Following config should change set -additional_arg JVM option and should change the workspace from ~/workspace to ~/.cache/jdtls.

require('lspconfig').jdtls.setup({
    init_options = {
        -- export DEFAULT_VMARGS so user can append additional args without
        -- having to manage default args?
        jvm_args = '-additional_arg',

        -- I like the idea of generic workspace, but better to have it working
        -- with init_options.workspace too I guess
        workspace = vim.env.HOME .. '/cache/.jdtls'
    }
})

Describe potential alternatives you've considered

To export DEFAULT_VMARGS, a possible solution would be, moving DEFAULT_VMARGS to nvim-lsp-installer/servers/jdtls/defaults.lua.