Add support for replace-existing-checks parameter

[alhimik45]

Consul docs about parameter: https://www.consul.io/api-docs/agent/service#replace-existing-checks
It was added in Consul v1.6.1, but not presented in this library

[marcin-krystianc]

@alhimik45 Thanks for your contribution, your changes look good. I would just improve tests by replacing these two new test methods with single parametrised one:

        [Theory]
        [InlineData(false)]
        [InlineData(true)]
        public async Task Agent_ReRegister_ReplaceExistingChecks(bool replaceExistingChecks)
        {
            var svcID = KVTest.GenerateTestKeyName();
            var check1Name = svcID + "1";
            var check2Name = svcID + "2";
            var check3Name = svcID + "3";
            var registration1 = new AgentServiceRegistration
            {
                Name = svcID,
                Port = 8000,
                Checks = new[]
                {
                    new AgentServiceCheck
                    {
                        Name = check1Name,
                        TTL = TimeSpan.FromSeconds(15)
                    },
                    new AgentServiceCheck
                    {
                        Name = check2Name,
                        TTL = TimeSpan.FromSeconds(15)
                    }
                }
            };
            var registration2 = new AgentServiceRegistration
            {
                Name = svcID,
                Port = 8000,
                Check = new AgentServiceCheck
                {
                    Name = check3Name,
                    TTL = TimeSpan.FromSeconds(15)
                }
            };

            await _client.Agent.ServiceRegister(registration1);
            await _client.Agent.ServiceRegister(registration2, replaceExistingChecks: replaceExistingChecks);

            var checks = await _client.Agent.Checks();

            if (replaceExistingChecks)
            {
                Assert.DoesNotContain(check1Name, checks.Response.Values.Select(c => c.Name));
                Assert.DoesNotContain(check2Name, checks.Response.Values.Select(c => c.Name));
            }
            else
            {
                Assert.Contains(check1Name, checks.Response.Values.Select(c => c.Name));
                Assert.Contains(check2Name, checks.Response.Values.Select(c => c.Name));
            }

            Assert.Contains(check3Name, checks.Response.Values.Select(c => c.Name));

            await _client.Agent.ServiceDeregister(svcID);
        }

[marcin-krystianc]

LGTM 👍