Adding new methods to manage the path params map

[franz1981]

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I've fallen into this while working on improving allocations for Quarkus:

• quarkus register several routes
• some of these routes, while iterated to find a match, hit https://github.com/vert-x3/vertx-web/blob/ed413097ca51eedc18db827c92f1514565e9b9b0/vertx-web/src/main/java/io/vertx/ext/web/impl/RouteState.java#L1225-L1230
• calling pathParams, while still null cause allocating it

And, at the same time, most of the time, the path params are just 1 or 2; meaning that the allocated map doesn't need to have an initial capacity that big.