Unify Url class to never have leading `/` in the path

[antkmsft]

There's this

azure-sdk-for-cpp/sdk/core/azure-core/inc/azure/core/url.hpp

Lines 152 to 159 in 868a14f

	void AppendPath(const std::string& encodedPath)
	{
	if (!m_encodedPath.empty() && m_encodedPath.back() != '/')
	{
	m_encodedPath += '/';
	}
	m_encodedPath += encodedPath;
	}

and this

azure-sdk-for-cpp/sdk/core/azure-core/src/http/url.cpp

Lines 218 to 226 in 9271d13

	if (!m_encodedPath.empty())
	{
	if (!relative)
	{
	if (m_encodedPath[0] != '/')
	{
	url += "/";
	}
	}

and also this

azure-sdk-for-cpp/sdk/core/azure-core/src/http/url.cpp

Lines 80 to 84 in ac75bd5

	if (*urlIter == '/')
	{
	++urlIter;
	auto const pathIter = std::find(urlIter, url.end(), '?');

and then this

azure-sdk-for-cpp/sdk/core/azure-core/src/http/curl/curl.cpp

Lines 725 to 732 in 7fe547b

	if (!m_httpProxy.HasValue() || request.GetUrl().GetScheme() == "https")
	{
	url = "/" + request.GetUrl().GetRelativeUrl();
	}
	else
	{
	url = request.GetUrl().GetAbsoluteUrl();
	}

Which, if you do

Url url("http://www.microsoft.com");
url.AppendPath("/path");

will make no difference when you call url.GetAbsoluteUrl(), but should you call GetRelativeUrl(), will have the leading slash. It will result in GET //path HTTP 1.1 instead of GET /path HTTP 1.1 when using curl transport.
WinHTTP apparently corrects for this, so the bug becomes platform-specific. Servers don't like the //, and may return 404 (FWIW, typespec's Spector test server does, and I think it won't be alone).

What I think we should do, is fix inside the Url::AppendPath() - add if (existingPath.empty() && pathThatWeReAboutToAdd.startsWith('/')) { pathThatWeReAboutToAdd.dropLeading('/'); }
This will be in line with #5187, making the approach consistent in a case when the existing path was empty. ALso, this will be consistent with the Url::Url(std::string) constructor which does parsing and drops the / before parsing path.

Because otherwise, why would it be ok to

Url url("http://www.microsoft.com/existingPath/");
// always safe to have leading '/' when appending
url.AppendPath("/newPath"); // GetRelativeUrl() => "existingPath/newPath"

but not

Url url("http://www.microsoft.com/");
// suddenly NOT safe to have leading '/' when appending ?!
url.AppendPath("/newPath"); // GetRelativeUrl() => "/newPath", should be "newPath"

cc @LarryOsterman, if maybe I am missing something, and you have another thoughts?

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